1. Field of the Invention
The current inventions pertains to an improved method for initially locating the position of the sealing head on a machine for making thermoplastic bags. The method is also applicable to all linear numerically controlled drive systems.
2. Prior Art
Traditional machines for the manufacture of thermoplastic bags usually employ a mechanical drive system having a rotating main shaft for forwarding the folded plastic film to a sealing and severing station and for actuating the sealing and severing head. This coupled drive system frequently used a crank and slider mechanism in conjunction with a clutch/brake apparatus for forwarding the material to the sealing station and a cam mechanism to drive the sealing head. Both the crank and slider mechanism and the cam mechanism for driving the head are operated from the main drive shaft. See, for example U.S. Pat. No. 2,947,345. In these types of machines, each rotation of the main drive shaft results in the production of one bag, with approximately one-half of the period of the main drive shaft rotation available for forwarding the material to the severing and sealing station, with the remainder of the shaft rotation available for raising, holding, and lowering the sealing head onto the thermoplastic material.
In an effort to improve the rate of bag production, the limiting item believed to be the time available to forward the material to the sealing station, bag machines employing servo-drive system for forwarding the material to the sealing and severing station became common. One such machine is the model 3800 Polybag Machine, marketed by the assignee of the present invention. This machine, while using a servo drive system to forward the material to the sealing and severing station, retained a cam driven sealing head operated from the rotating main drive shaft. This machine achieved a marked improvement in bag production rate by more efficiently utilizing the overall cycle time. Specifically, the time available for forwarding material to the sealing and severing station was no longer limited to one half of the period of rotation of the main drive shaft, a constraint previously associated with the crank and slider mechanism. Other manufacturers also offered such types of bag making machines.
More recently, however, in attempts to further raise bag production rates, cam or crank drives for the sealing head operated from the rotating main drive shaft have begun to be replaced by stepper or servo driven sealing heads. See U.S. Pat. Nos. 5,002,522 and 5,230,688. As shown in both of the above referenced patents, despite the separate drive motors for the sealing head, the preferred mechanism for driving the head has remained one based on rotary motion, either a crank mechanism, U.S. Pat. No. 5,002,522, or a cam mechanism, U.S. Pat. No. 5,230,688. Whatever the success of these mechanisms, however, they limit the extremes of head motion to a fully down (sealing) position, and a fully raised position. In practice, a third head position, further away from the web than the usual fully raised position, is desirable.
In an effort to overcome the limitations of travel, the inventor of the present application has filed another patent application Ser. No. 08/337,282 filed Nov. 10, 1994, covering a linear drive system for a sealing head. During development of this head drive system, it became clear that some method for initially locating the position of the linear drives and head was necessary, as currently available numerical control units typically employ volatile memories. Any time that the machine would be fully powered down, as is currently required under OSHA regulations for maintenance purposes, the numerical processing unit would loose track of current position. This loss of position was usually overcome by introducing a limit switch into the path of travel of the linear device, so that, upon re-energization of the machine, the linear device could establish a known position by making the limit switch.